As well known, in the packaging industry, both for food and pharmaceutical applications, it is necessary to limit the permeability of the packages both to oxygen and to other gas species, to protect with time the quality of the contents.
A system for measuring permeability of containers to oxygen is described in US2005076705. In this document a description is given on how to measure with special sensors a volume variation of a gas permeated into the container, in two different instants and according to the perfect gas law. Then a described equation is integrated with time obtaining a volumetric variation of gas with time in the container. Before carrying out the two volumetric measurements, it is necessary to await the end of a transient phase with subsequent long and expensive waits.
In another common method a container to be analysed is put in a test chamber. In the test chamber the container is suitably sealed and insulated, except from an inlet duct and an outlet duct through which a gaseous carrier, normally nitrogen, is caused to flow into the container. In the test chamber, out of the container, a pure gas sample is delivered, normally oxygen or carbon dioxide, or a mixture comprising the gas carrier and the gas sample. The gas sample can permeate into the container through the walls of the container same. The gas carrier that flows in the container has a double function of maintaining in the container the same pressure as in the test chamber, normally atmospheric pressure, for balancing the forces and avoiding that the container collapses, as well as for carrying the permeated gas towards a sensing device for detecting its concentration.
The gas carrier that flows out from the container reaches a sensing device, normally an electrochemical cell or a infrared detector. The sensing device, once started the measure, reveals a variable concentration value of the permeated gas in the flow of the gaseous carrier. By keeping constant both the gas sample flow in the test chamber and the gas carrier flow in the container, the sensing device follows a chart that converges to a steady condition, i.e. a condition in which the permeated gas measured concentration is constant with time. The measured value, once converged, represents the gas flow permeating into the container i.e a value proportional to the permeability of the container.
The same method can be used for measuring the permeability of low permeability closure elements for containers, such as corks or caps. The container is arranged in the test chamber in a similar way as above described, sealed by the closure element. In the walls of the container inlet and outlet passages are artificially made, normally sealed with epoxy resin, for the flow of the gaseous carrier.
The problem of this system is the waiting time necessary for achievement of the convergence, which can in some cases be very long, for example for corks even for weeks, with consequent high process costs.